chore: switch database engine to sqlite (fixes #9954) (#9965)

Switch the database from LevelDB to SQLite, for greater stability and
simpler code.

Co-authored-by: Tommy van der Vorst <tommy@pixelspark.nl>
Co-authored-by: bt90 <btom1990@googlemail.com>

internal/db/sqlite/db_mtimes_test.go

@@ -0,0 +1,54 @@
+// Copyright (C) 2025 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package sqlite
+
+import (
+	"testing"
+	"time"
+)
+
+func TestMtimePairs(t *testing.T) {
+	t.Parallel()
+
+	db, err := OpenTemp()
+	if err != nil {
+		t.Fatal()
+	}
+	t.Cleanup(func() {
+		if err := db.Close(); err != nil {
+			t.Fatal(err)
+		}
+	})
+
+	t0 := time.Now().Truncate(time.Second)
+	t1 := t0.Add(1234567890)
+
+	// Set a pair
+	if err := db.PutMtime("foo", "bar", t0, t1); err != nil {
+		t.Fatal(err)
+	}
+
+	// Check it
+	gt0, gt1 := db.GetMtime("foo", "bar")
+	if !gt0.Equal(t0) || !gt1.Equal(t1) {
+		t.Log(t0, gt0)
+		t.Log(t1, gt1)
+		t.Log("bad times")
+	}
+
+	// Delete it
+	if err := db.DeleteMtime("foo", "bar"); err != nil {
+		t.Fatal(err)
+	}
+
+	// Check it
+	gt0, gt1 = db.GetMtime("foo", "bar")
+	if !gt0.IsZero() || !gt1.IsZero() {
+		t.Log(gt0, gt1)
+		t.Log("bad times")
+	}
+}